Conventionally, there are various proposals on a method of improving tone quality of output speech of a device for outputting a speech signal. One of relatively simple methods thereof is a method of applying low pass filter processing and high pass filter processing to an input speech signal.
This type of tone quality adjustment device allows an input speech signal to pass through a low pass filter and a high pass filter, controls gains of an output signal and input speech signal of each filter and adds up all signals. At this time, by arbitrarily setting the gain for each filter output and the gain for the input speech signal, it is possible to arbitrarily emphasize a tone of a desired frequency band.
For example, to emphasize a tone in a low-frequency region (so-called low tone), the gain for the output signal of the low pass filter may be increased. On the other hand, to emphasize a tone in a high-frequency region (so-called high tone), the gain for the output signal of the high pass filter may be increased.
As filters used for this type of tone quality adjustment device, an IIR (Infinite Impulse Response: infinite length impulse response) filter and FIR (Finite Impulse Response: finite length impulse response) filter are often used. Among these filters, the FIR filter has the following advantages. First, since extreme values of a transfer function of the FIR filter exist only at the origin of the z-plane, the circuit is always stable. Second, a completely accurate linear phase characteristic can be realized.
The basis for these IIR filter and FIR filter is a low pass filter, and other filters such as high pass filter, band pass filter, band erasure filter, etc., are derived from the low pass filter by carrying out processing such as a frequency conversion. The frequency conversion processing here carries out a convolutional calculation using a window function and Chebyshev approximation method, etc., based on the ratio with respect to a sampling frequency and cutoff frequency to obtain a transfer function of the filter and further replaces the transfer function by a frequency component.
However, the above described conventional tone quality adjustment device filter designing method requires high-level expert knowledge of frequency conversion, etc., which results in a problem that it is not possible to design a tone quality adjustment device easily. On the other hand, the frequency conversion using a window function and Chebyshev approximation method involves very complicated calculations. For this reason, there has been a problem that realizing the frequency conversion by software results in a heavy processing load, while realizing the frequency conversion by hardware increases the scale of the circuit.
The present invention has been implemented to solve such problems and it is an object of the present invention to make it possible to simply design a tone quality adjustment device using a FIR digital filter.